The first mission, set to lift-off on Monday 9 June after bad weather delayed the launch, comes within a week of the despatch of Europe's first Mars mission.

The US is sending two rovers to different sites on Mars to study the geology of rocks.

The "robot geologists" are hunting for chemical signatures that would confirm water once existed on the surface of Mars.

The new generation robots are bigger and more powerful versions of Sojourner, the micro-rover that was part of Nasa's Mars Pathfinder mission in 1997.

Each rover is about the size of a small car and is mounted on six wheels. According to Nasa, they can see sharper images and examine rocks better than anything else that has landed on Mars.

Sojourner managed to roam around an area about the size of a football pitch during its stay on the Red Planet.

The Mars Exploration Rovers (Mer) should be able to trek longer distances across the rocky terrain, about 40 metres each Martian day.

Unlike the mini Sojourner, the new generation rovers will carry all their instruments with them. They will be commanded to go to rock and soil sites of interest by scientists on Earth, some 460 million kilometres away.

Historic landing

Only three Nasa space craft have ever touched down successfully on Mars before: the Viking landers of 1976 and Mars Pathfinder.

If all goes well, the rovers will arrive at the Red Planet in January 2004. They are heading for two places that appear to have once been flooded by water.

The Viking landers explored the Martian surface in 1976

One is the ancient Gusev crater, which may once have contained a lake; the other a flattish plain, Meridiani, which could have been peppered by hot springs.

The British-built Beagle 2 lander, part of the European Space Agency's Mars Express mission, will land at a third location, the Isidis Basin, around the same time.

It is a fixed lander and cannot move although it has a mole and a robotic arm to dig into rocks and soil to collect samples.

Beagle 2 will analyse samples for chemical signs of life on Mars. This has only been done once before, in the 1970s, by Nasa's Viking landers and proved unsuccessful. Nasa has decided not to repeat specific biology experiments this time round.